The present invention relates to an ink ribbon cassette for use in a thermal transfer printer, and more particularly to an ink ribbon cassette for use in a thermal transfer printer suitable for an aligned winding of an ink ribbon in the ink ribbon cassette to maintain the stability of tension force in the ink ribbon of a reciprocating ink ribbon cassette.
A disadvantage of an ink ribbon in the ink ribbon cassette used in a conventional thermal transfer printer resides in the fact that, because an ink of the ink ribbon in the ink ribbon cassette is completely transferred to a thermal transfer printing paper in only one printing operation and cannot be reused, the ink ribbon increases the overall operational costs of the thermal transfer printer.
To avoid the above described disadvantage, a thermal transfer printer was proposed in Japanese Utility Model Laid-Open No. 194042/1983, wherein the thermal transfer printer has a reversing mechanism for reversing a driving direction of an ink ribbon and also a vertical movement mechanism for a thermal head whereby it is possible to print in both the forward direction and the backward direction with two rows, that is, an upper row and a lower row of the ink ribbon in the ink ribbon cassette. However, this thermal transfer printer is structurally complex because both a reversing mechanism for the ink ribbon and a vertical movement mechanism for the thermal head are necessary.
Furthermore, for the thermal transfer printer to reciprocatingly print the moving distances of the ink ribbon to go in a forward direction and return in a backward direction must be equal, and hence the ink ribbon cannot stop moving and printing in the same line of the ink ribbon. This is not economical in terms of the consumption of the ink ribbon in the ink ribbon cassette.
When printing in the thermal transfer printer is carried out in a pair of upper and lower rows of the ink ribbon only by reversing the ink ribbon cassette without both the reversing mechanism in a driving direction of the ink ribbon and the vertical movement mechanism of the thermal head in the thermal transfer printer itself, the thermal transfer printer can unidirectionally print.
The effective length of used ink ribbon received in the ink ribbon cassette in such a thermal transfer printer can be twice that of previous ones, so that a lower operational cost can be realized by the user of the thermal transfer printer.
However, there are technically difficult problems in the printing operation for the thermal transfer printer having the ink ribbon in plural rows such as two upper and lower rows using the same ink ribbon.
These problems are caused by factors such as a "partial printing" phenomenon in which wrinkles are caused by the ink ribbon shifting down during the winding start of the ink ribbon in which a large tension force ratio exists, a "background dirty" phenomenon in which tail dragging is caused by the reversal between the winding force tension (T.sub.o) and the back tension force (T.sub.i) (the brake force against the taking up force) just before completion of printing and further a reverse rotation brake torque of the winding take-up shaft which occurs after one line printing of the thermal head, and a head touch smudge caused by slack ink ribbon, etc.
The tension force ratio (T.sub.o /T.sub.i) is a ratio of a winding tension force (T.sub.o) to a compound brake force (T.sub.i). The compound brake force (T.sub.i) is an outer peripheral brake force and a friction resistance force of a supply shaft, in which an ink ribbon supply core is engaged with, namely, a brake force by a constant brake torque.
As stated above, when printing by the thermal transfer printer is carried out in a pair of upper and lower rows of the ink ribbon only by the reverse mechanism of the ink ribbon cassette without both the reversing mechanism in the driving direction of the ink ribbon and the vertical movement mechanism of the thermal head in the thermal transfer printer itself, the thermal transfer printer can utilize a unidirectional printing method, and further the effective length of used ink ribbon in the thermal transfer printer can be twice that of previous ones.
However, the ink ribbon fold, the printing rub, and the partial printing etc., which are caused respectively by the "ink ribbon shifting down" phenomenon of the ink ribbon become serious problems. In particular, the ink ribbon fold, which is caused by the overlapping portion of the ink ribbon in a special pattern, such as a mesh pattern, by the "ink ribbon shifting down" phenomenon happens often and becomes a serious problem of utmost importance. We have experimentally ascertained that this ink ribbon fold phenomenon does not occur when the back tension force is increased.
However, when the back tension force increases, the tension force ratio relationship is reversed rapidly before the printing operation is complete, as a result of which the "background dirty" phenomenon such as tail dragging starts to occur. As a way of suppressing the "ink ribbon shifting down" phenomenon, it has been proposed to provide a felt member on a plate spring member, which is disposed in the vicinity of the thermal head so that the ink ribbon is pressed down at the vicinity of the thermal head.
However, experimentation has determined that, even if the ink ribbon is merely pressed down, the "ink ribbon fold" phenomenon caused by the "ink ribbon shifting down" phenomenon is not prevented. Such an "ink ribbon shifting down" phenomenon is caused by the imbalance of the ink ribbon tension force. The tension force runs from the take up side to the supply side through the ink ribbon which is one having no attached portion with the glaze portion of the thermal head.
With only the fixed or constant back tension force, the distance from the fixed or constant back tension force portion to the winding outer peripheral portion of the supply ink ribbon is too long, and an additional ink ribbon tension force imbalance caused by the slack is created. The looseness of the supply core and the inclination of the supply core also add to the imbalance.
The "ink ribbon shifting down" phenomenon can be prevented to a certain extent by adding a fixed or constant brake force with the supply core portion. In other words, a felt-like seat brake member for adding brake force to the take-up core or supply core or a spring member mounted on the ink ribbon outer peripheral portion for pressing the ink ribbon with the adhesion member having a felt friction member as a felt member can be provided. Taking account of the scattering of the take up shaft torque or the scattering of the ink ribbon back tension force, it is necessary to enlarge still more the use range of the ink ribbon tension force ratio.
Therefore, with only the above stated three countermeasures against the prevention of the "ink ribbon shifting down" phenomenon, the ink ribbon tension force ratio cannot be maintained constant over the whole printing range. Further, the felt-like seat brake structure is made up to sandwich the felt member in the clearance between the ink ribbon cassette interior upper and lower surfaces and the supply core, so that large scattering of the ink ribbon back tension force exists, and the ink ribbon back tension force becomes further unstable.
Herein, generation of the wrinkles, which occurs by the "ink ribbon shifting down" phenomenon, will be explained. As compared with the thermal transfer printer in which the central part of the ink ribbon is used for printing, when the thermal transfer printer prints in plural rows, such as the upper and lower two rows, the center of the ink ribbon is not at the printing center of the ink ribbon. As a result, the stress distribution acting on the ink ribbon during ink ribbon travel differs from the upper row of the ink ribbon to the lower row of the ink ribbon.
At the ink ribbon upper row printing, the ink ribbon is shifted down at the thermal head portion to the lower direction, thereby the ink ribbon wrinkles occur. This phenomenon occurs easily in a mesh pattern during the return passage printing process, when the ink ribbon back tension force is smaller or the taking up tension force is excessive. The reasons will be explained as follows.
The thickness of the ink ribbon is very thin, such as about 6-7 .mu.m. When stress occurs in the interior portion of the ink ribbon, the longitudinal force becomes the predominant one. The ribbon thickness at the ink used up portion or the ink omitted portion is reduced to about 3 .mu.m thickness by the omission of the ink. When the ink ribbon tension force is applied at the ink used up portion, ink ribbon local elongation at the ink used up portion occurs by virtue of the heating of the thermal head. As a result, the ink used up portion of the ink ribbon is expanded and absorbs the inner strain of the ink ribbon.
Due to localized decrease of the elongation rigidity by the expansion of the ink ribbon and the thickness reduction in the ink ribbon, ink ribbon tension force cannot be exerted at the ink used up portion of the ink ribbon. Consequently the ink ribbon tension force distribution in the width direction of the ink ribbon is divided at the ink used up portion of the ink ribbon. In other words, wrinkles occur at the ink ribbon because the ink ribbon tension force distribution changes rapidly in the width direction of the ink ribbon. The ink ribbon wrinkles occur frequently at the line or boundary between an ink ribbon tension force flowing portion and a no tension force portion.
The generation of the ink ribbon wrinkles, which occurs by the "ink ribbon shifting down" phenomenon, will be explained with reference to FIGS. 1 and 2.
FIG. 1 is a side view taken from the back portion of the thermal head 103. It shows schematically the state where the thermal head 103 moves to the right (the direction of the arrow beneath the thermal head 103) when printing is being performed at the upper row 108 of the ink ribbon 112. The ink ribbon 112 has a width R. The diagonal lines indicate an unused portion of the ink ribbon 112, and the white portion of the ink ribbon 112 indicates the used up portion or the left out portion after the transfer printing operation on the thermal transfer printing paper.
The tension force distribution acting on the ink ribbon 112 of the above structure is shown in FIG. 2 in which lower row 110 dot printing is performed by using the thermal head 103. Such printing, for example a special mesh pattern printing, easily causes ink ribbon wrinkles to occur.
In the ink ribbon 112 being pressed by the thermal head 103, the taking up tension force (T.sub.o) balances the back tension force (T.sub.i) and the ink ribbon friction braking force (T.sub.H) according to the pressing force of the thermal head 103. The taking up tension force (T.sub.o) is divided into two tension forces which are the lower tension force (T.sub.o1) and the upper tension force (T.sub.o2). The upper tension force (T.sub.o2) balances with the ink ribbon friction brake force (T.sub.H).
At the used up portion (A) of the ink ribbon 112, the ink of the ink ribbon 112 is omitted and the thickness of the ink ribbon 112 is reduced from about 6 .mu.m to about 3 .mu.m. Further, by the heating of the thermal head 103, the taking up tension force (T.sub.o) is divided into the upper portion tension force (T.sub.o2) and the lower portion tension force (T.sub.o1) with the used up portion (A) acting as the boundary portion. As the lower tension force (T.sub.o1) is divided from the used up portion (A), the lower tension force (T.sub.o1) flows through the lower side of the thermal head portion and balances with the back tension force (T.sub.i).
As a result, the ink ribbon tension force becomes nearly zero at a region (So) of the ink ribbon 112 on the upstream side of the thermal head 103. Owing to the foregoing ink ribbon wrinkles occur easily at a boundary line portion (K) because the flow of the tension force in the ink ribbon 112 changes rapidly.